FAUNAL GRADIENTS IN SURFACE AND SUBSURFACE SHELLY ACCUMULATIONS FROM A RECENT CLASTIC TIDAL FLAT, BAHIA-LA-CHOYA, NORTHERN GULF-OF-CALIFORNIA, MEXICO

Do the shelly remains already accumulated at depth reflect the faunal composition and environmental gradients evident in surface deposits, o r do many years of selective destruction and post-mortem transportatio n limit the spatial resolution possible in studies of nearshore marine faunas? We sampled shelly molluscan remains at the surface and at dep th along two 2 km transects across the intertidal zone of Bahia la Cho ya, Sonora, Mexico. At each site we collected surface samples from the upper 5 cm and depth samples from a ubiquitous shelly layer approxima tely 25 cm below the surface. Radiocarbon dates indicate that the samp les differ by approximately 800 years in age. We used the 25 most comm on species in subsequent analyses. Faunal composition and distribution patterns vary systematically among the four sets of samples. Cluster analysis and multidimensional scaling (MDS) show that subsurface trans ects reflect species distributions of their surface counterparts more closely than species distributions in surface (or depth) transects res emble each other. MDS of samples also highlights a distinctive gradien t in species distributions: Dimension 1 consistently expresses the dep th gradient across the tidal flats. Dimension 2 appears to represent s ubtle changes in substrate mobility. These results suggest that, despi te the potential for shell transport and extensive time-averaging, sur face heterogeneity is accurately recorded in the subsurface shelly bed . Environmental gradients may have remained constant during the simila r to 500-1300 years since deposition of the subsurface bed, reworking may have completely mixed the sedimentary interval, or both. Spatial r esolution of approximately 100 m may be possible in intertidal flat ha bitats in the stratigraphic record.